1. Field of the Invention
The present invention is directed to intermediates which can be converted into podophyllotoxin and related compounds, which are known antineoplastic agents. More specifically, this invention relates to an efficient total synthesis of podophyllotoxin. Additionally, the present invention provides processes for the preparation of such intermediates, and processes for the conversion of the intermediates into known intermediates which are readily converted into podophyllotoxin and related compounds.
2. Description of the Prior Art
Podophyllotoxin (I), a known lignan lactone isolated from several species of Podophyllum, is a potent cytotoxic agent. Numerous other related compounds having the characteristic aryltetralin ring structure, either naturally occurring or derived from some naturally occurring compounds are known. Some of these compounds possess antineoplastic activity while others are useful for conversion to compounds having such activity. Podophyllotoxin is an important intermediate for the production of the antitumor agent etoposide and its analogues. Podophyllotoxin has the structure shown below: ##STR1##
The key molecular features of podophyllotoxin are (1) presence of the C2-C3 trans lactone, and (2), a cis relationship between the C1 and C2 substituents. For the synthesis of etoposide, the C4 hydroxy group can be either in the .alpha. (podophyllotoxin) or the .beta. (epipodophyllotoxin) orientation because in the glycosidation step only the C4 .beta. glycoside is obtained.
In J. Org. Chem., 31, 4004-4008 (1966), W. J. Gensler and C. D. Gatsonis describe the completion of the total synthesis of podophyllotoxin through the epimerization by enolate quenching of the O-tetrahydropyranyl derivative of picropodophyllin. However, this epimerization does not proceed to completion, and separation of an about 45:55 mixture of podophyllotoxin and picropodophyllin is required. Picropodophyllin which is the cis-lactone isomer of podophyllotoxin has the structure: ##STR2##
In J. Org. Chem., 46, 2826-2828 (1981), A. S. Kende et al. report on an improved total synthesis of podophyllotoxin in 12 steps with an overall yield of 4.5% from piperonal. However, the Kende synthesis requires the preparation and then the subsequent epimerization of picropodophyllin similar to the above-mentioned Gensler synthesis.
In J. Am. Chem. Soc., 103, 6208-6209 (1981), D. Rajapaksa and R. Rodrigo report a new synthesis of podophyllotoxin which avoids the thermodynamic hurdle present in the conversion of picropodophyllin to podophyllotoxin as was previously described in the above-mentioned references of Gensler et al. and Kende et al. However, the synthesis requires the preparation of a bicyclic precursor and a satisfactory yield can be achieved only by recycling procedures.